Multiplexing systems can be defined as digital based electronic schemes for sequentially accessing a variety of remote stations or nodes. Such systems comprise a central control unit connected to a plurality of remote nodes via a bi-directional transmission line. The signals communicated between the central control unit and the remote nodes take the form of electronic bits of information which define either a logic "0" or "1" binary state.
In a time division multiplex system the communicated data is defined by time windows also known as slots or frames. Each bit position within a time window has a predefined meaning which can change based on the addition of complex flagging bits or patterns that may sandwich the data signal bits. An additional or different kind of flag bit may also be included to indicate that more signal bits are coming for a particular frame and/or that selected bit positions are to be redefined.
U.S. Pat. No. 4,311,986 issued to Yee describes a multiplex system in which pulse width modulation is utilized to create a unique address for each remote station in the system. The central processor sends out interrogation pulses of a controlled width, wherein the width identifies the specific remote station which is the intended recipient of the pulse. Each remote station, when it receives an interrogation pulse of the appropriate width, responds by supplying a return pulse on the bus with a delay that encodes the desired information. In the Yee system, the variable width interrogation pulse identifies the remote station by the width of the pulse.
In the system with which the present invention finds utility, the remote stations may be addressed either directly or sequentially. In the latter case, such addressing proceeds in order from a first one of the remote stations to a last one of the remote stations. When the stations are addressed sequentially, the data transmitted by the control unit includes a first number of data digits and the remote stations count the transmissions from the control unit. Each station recognizes when the transmission count equals its assigned address. When a remote station is to be addressed directly, the data transmitted by the control unit includes a second number of data digits which is equal to the first number plus a third number of address data digits. Thus, transmissions from the control unit should include only either the first or the second number of digits. Each of the digits must have a defined set of characteristics in order to be valid and the remote stations examine the received digits as part of the validation routine. However, due to the presence of noise on the transmission line, one or more digits of a data transmission are occasionally "lost". Further, noise on the transmission line can also occasionally appear to be valid digits.
It is an object of the present invention to provide an arrangement in each of the remote stations which reacts to the presence of an incorrect number of received digits in a different manner depending upon the number of digits received.